1. Field of the Invention
The present invention relates to detergent compositions which have improved degradation resistance to cotton fabrics. More particularly, the present invention relates to detergent compositions containing a combination of exo-cellobiohydrolase I type cellulase components and endoglucanase components wherein the exo-cellobiohydrolase I type cellulase components are enriched relative to the endoglucanse type cellulase. Such detergent compositions provide excellent cleaning especially of cotton garments while also providing substantially reduced degradation of the cotton fabric in the garment.
2. State of the Art
Cellulases are known in the art as enzymes that hydrolyze cellulose (.beta.-1,4-glucan linkages) thereby resulting in the formation of glucose, cellobiose, cellooligosaccharide, and the like. While cellulases are produced in fungi, bacteria and the like, those produced by fungi have been given the most attention because fungi typically produce a complete cellulase system capable of degrading crystalline forms of cellulose and such cellulases can be readily produced in large quantities via fermentation procedures. In fact, as noted in "Methods in Enzymology", 160, 25, pages 234 et seq. (1988) and elsewhere, a cellulase system produced by a given microorganism is comprised of several different enzyme components including those identified as exo-cellobiohydrolases (EC 3.2.1.91) ("CBH"), endoglucanases (EC 3.2.1.4) ("EG"), .beta.-glucosidase (EC 3.2.1.21) ("BG"). Moreover, these classes can be further separated into individual components. For example, multiple CBHs and EGs have been isolated from a variety of bacterial and fungal sources including T. reesei which contains 2 CBHs, i.e., CBH I and CBH II, and at least 2 EGs, i.e., EG I and EG II. The ratio of CBH I components to EG components (including all of the EG components) in naturally occurring cellulases does not exceed about 5:1. For example, see Brown et al., Genetic Control of Environmental Pollutants, Gilbert S. Omenn Editor, Chapter--"Microbial Enzymes and Ligno-Cellulase Utilization", Hollaender Publishing Corp. Variations in this ratio can result from the use of different microorganisms, depending upon the characteristics of the strain, but in any event such ratios still do not exceed about 5:1.
The complete cellulase system comprising CBH, EG and BG is required to efficiently convert crystalline cellulose to glucose. Isolated components are far less effective, if at all, in hydrolyzing crystalline cellulose. Moreover, a synergistic relationship is observed between the cellulase components. That is to say the effectiveness of the complete/whole system is significantly greater than the sum of the contributions from the isolated components. It has also been suggested by Wood, "Properties of Cellulolytic Systems", Biochem. Soc. Trans. 13, 407-410 (1985), that CBH I and CBH II derived from either T. reesei or P. funiculosum synergistically interact in solubilizing cotton fibers. On the other hand Shoemaker et al., Bio/Technology, October 1983, discloses that CBH I (derived from T. reesei), by itself, has the highest binding affinity but the lowest specific activity of all forms of cellulase.
The substrate specificity and mode of action of the different cellulase components varies from component to component which may account for the synergy of the combined components. For example, the current accepted mechanism of cellulase action is that endoglucanase components first break internal .beta.-1,4-glucosidic bonds in regions of low crystallinity of the cellulose thereby creating chain ends which are recognized by CBH components. The CBH components bind preferentially to the non-reducing end of the cellulose to release cellobiose as the primary product. .beta.-Glucosidase components act on cellooligosaccharides, e.g., cellobiose, to give glucose as the sole product.
Cellulases are also known in the art to be useful in detergent compositions either for the purpose of enhancing the cleaning ability of the composition or as a softening agent. When so used, the cellulase will degrade a portion of the cellulosic material, e.g., cotton fabric, in the wash which in one manner or another facilitates the cleaning and/or softening of the cotton fabric. While the exact cleaning mechanism of cotton fabrics by cellulase is not fully understood, the cleaning of cotton fabrics by cellulase has been attributed to its cellulolytic activity. Thus, for instance, U.S. Pat. No. 4,822,516 discloses that detergent compositions containing a cellulase having low activity on highly crystalline cellulose and high activity on low crystalline cellulose possesses good detergency and a low degree of damage on cotton garments. As noted by Wood, supra., the presence of CBH components is the distinguishing feature of cellulases that are able to degrade crystalline cellulose. Accordingly, these references would suggest that CBH components are in some form involved in the degradation of cotton fabric.
However, regardless of its cleaning and/or softening mechanism(s), the use of cellulases in detergent compositions is complicated by the fact that exposure of cotton garments to cellulase results in partial degradation of the cotton fabric in these garments. After repeated washing and drying, the integrity of the cotton garment is compromised resulting in the tearing, weakening and/or thinning of the cotton garment. When its integrity has been so compromised by repeated exposure to cellulase containing detergents, the cotton garment is no longer of any practical utility. Needless to say, such degradation greatly impairs the commercial utility of cellulases in detergent compositions. Accordingly, cellulase compositions have been sought which possess reduced cotton degradation while retaining enhanced cleaning capabilities.
Accordingly, it is an object of this invention to develop a detergent composition containing cellulase which is resistant to degrading cotton fabrics. It is a further object of this invention that such detergent compositions provide excellent cleaning of such cotton fabrics. These and other objects are achieved by the present invention as evidenced by the attached summary of the invention, detailed description of the invention and claims.